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Am Fam Physician. 2022;106(2):137-147

Patient information: See related handout on how to protect yourself from ticks, written by the authors of this article.

This clinical content conforms to AAFP criteria for CME.

Author disclosure: No relevant financial relationships.

Arthropods, including insects and arachnids, significantly affect humans as vectors for infectious diseases. Arthropod bites and stings commonly cause minor, usually self-limited reactions; however, some species are associated with more severe complications. Spider bites are rarely life-threatening. There are two medically relevant spiders in the United States. Widow spider (Latrodectus) envenomation can cause muscle spasm and severe pain that should be treated with analgesics and benzodiazepines. Antivenom is not widely available in the United States but may be considered for severe, refractory cases. Recluse spider (Loxosceles) bites are often overdiagnosed, should be treated supportively, and only rarely cause skin necrosis. Centruroides scorpions are the only medically relevant genus in the United States. Envenomation causes neuromuscular and autonomic dysfunction, which should be treated with analgesics, benzodiazepines, supportive care, and, in severe cases, antivenom. Hymenoptera, specifically bees, wasps, hornets, and fire ants, account for the most arthropod-related deaths in humans, most commonly by severe allergic reactions to envenomation. In severe cases, patients are treated with analgesia, local wound care, and systemic glucocorticoids. Diptera include flies and mosquitoes. The direct effects of their bites are usually minor and treated symptomatically; however, they are vectors for numerous infectious diseases. Arthropod bite and sting prevention strategies include avoiding high-risk areas, covering exposed skin, and wearing permethrin-impregnated clothing. N,N-diethyl-m-toluamide (DEET) 20% to 50% is the most studied and widely recommended insect repellant.

Arthropods, comprised of insects and arachnids, account for up to 1 million emergency department visits annually in the United States.1 Arthropods' most significant effects on humans are as vectors for infectious diseases, and direct effects of their bites and stings are typically only a self-limited nuisance (Table 125). Local effects can be treated symptomatically with topical or oral antihistamines, calamine lotion, topical corticosteroids, cold compresses, or, in severe cases, systemic glucocorticoids. There are limited data to support one treatment over another.

ArthropodClinical manifestationsBody distributionGeographic distributionAssociated vector-borne diseases
Bed bugs (Cimex lectularius and Cimex hemipterus)Simple: erythematous, pruritic macules or papules, 2 mm to 5 mm
Complex: pruritic wheal around central punctum, urticaria, progressive bullous rash with secondary infection
All body regions, most commonly on the back or on warm or occluded regions of the bodyTemperate regions throughout North AmericaNone
Biting midges (Ceratopogonidae)Sharp burning pain with resulting papule, bite often not witnessedHead and neckThroughout North America, predominantly in coastal, marshy areasMansonellosis (Africa and tropical Americas)
Brown recluse spider (Loxosceles reclusa)Initially painless or occasional burning
Single, red papule with central pallor
Can become painful and necrotic
Upper arm, thorax, inner thigh (rolling onto spider in bed or trapping in clothing)Southern Midwest and Southwestern United StatesNone
Fleas (Ctenocephalides felis and Oropsylla montana)Erythematous pruritic maculopapular lesions, less than 1 cmTypically lower extremities (feet or ankles)Throughout United States; vectors of human pathogens primarily in western United States, Texas, HawaiiCat-scratch fever, flea-borne (murine) typhus, plague
Horseflies and deer flies (Tabanidae)Bites are deep, painful, and wounds tend to bleed after flies have leftMost commonly on exposed skin, but can bite through clothing; bites occur most often during the dayThroughout North AmericaAnthrax, loiasis, tularemia
Mites and chiggers (Sarcoptes scabiei and Trombicula spp. )Localized pruritic erythematous papules or burrowsTypically on constrictive clothing lines (beltlines, socks), wrists, between fingersThroughout United States in cool areas with high humidity and dense vegetationRickettsialpox, scrub typhus (Asian-Pacific region)
Mosquitoes (Culicidae)Intensely pruritic papules, sometimes result in wheal-and-flare reaction, can result in dramatic surrounding indurationExposed skinNortheast, South, West coastJapanese encephalitis, lymphatic filariasis, malaria, West Nile virus, St. Louis encephalitis, western equine encephalitis, eastern equine encephalitis, Venezuelan equine encephalitis, Ross River virus, Rift Valley fever virus, dengue, yellow fever, chikungunya, zika
Sandflies (Phlebotomus, Lutzomyia)Bites may be painless, mildly painful, or intensely pruritic; result in small papulesExposed skin; bites occur most often at nightThroughout North AmericaLeishmaniasis, sandfly fever, bartonellosis (Carrión disease), papataci fever (three-day fever)
Ticks (Ixodes scapularis, Dermacentor andersoni, Rhipicephalus sanguineus, Amblyomma americanum, Ornithodoros genus)Local: expanding erythematous patch initially homogenous then progressing with central clearing over a few days (erythema migrans) or macular rash
Systemic: neurologic (meningitis, facial palsy), musculoskeletal (arthralgias, myalgias) cardiovascular (temporary atrioventricular block)
Flulike symptoms, lymphadenopathy
Typically lower extremities (feet or ankles)Throughout United States (except Alaska) but most encountered in Eastern, Midwest, and Rocky Mountain statesLyme disease, Rocky Mountain spotted fever, Rickettsia disease, anaplasmosis, ehrlichiosis, babesiosis, Colorado tick fever, tickborne relapsing fever, tularemia
Widow spiders (Latrodectus)Asymptomatic or pinprick
Single, target lesion is classic, although not always present
Typically extremities, more often lower (accidentally encroaching on outdoor habitat)North AmericaNone

Spiders

Spider bites are blamed for many necrotic wounds, but most spiders are harmless to humans. Many suspected bites are caused by other conditions such as Staphylococcus aureus infection.6 Spider bites typically present as a solitary papule, pustule, or wheal, and require only local wound care, analgesics, and tetanus vaccine prophylaxis. The two spiders of most medical significance in the United States are widow (Latrodectus) and recluse spiders (Loxosceles).

WIDOW SPIDERS

Widow spiders include more than 30 species with worldwide distribution. They are medium-sized spiders (up to 4 cm), characterized by shiny, dark-colored bodies with ventral red or yellow abdominal demarcations2 (Figure 1). The black widow (Latrodectus mactans) is the most predominant of the five widow spiders in the United States. Widows are rarely found inside the home and are typically encountered in shady enclosed spaces outdoors (e.g., sheds, yard debris, gardening equipment). Most bites do not result in systemic envenomation, referred to as latrodectism, characterized by excess acetylcholine release.7 When present, symptoms of latrodectism include muscle spasm and diaphoresis starting at the involved extremity and migrating proximally. Patients can present with nausea, tachycardia, hypertension, restlessness, and, rarely, severe abdominal rigidity or chest pain, which can be mistaken for peritonitis or myocardial infarction.2 Infants and children typically present as inconsolable with generalized erythema and excessive drooling. Symptoms typically come in waves and continue for 48 to 72 hours but are rarely life-threatening.7,8

Envenomation severity can be graded, which helps determine treatment8 (Table 22,7,8). Mild presentations can be treated with nonopioid oral pain medications, but opioids are sometimes indicated for poorly controlled pain. Benzodiazepines are widely recommended to treat painful muscle spasms; however, no trial data support their use.2,8 Calcium and magnesium have demonstrated no benefit and are not recommended.2,8 Widow spider bites are rarely life-threatening, and the use of antivenom is controversial. Limited data have demonstrated decreased pain duration with antivenom, balanced with a risk of allergic reaction in up to 5% of patients.79 Antivenom is not widely available in the United States and is generally only considered in patients with severe symptoms not responsive to supportive care.

GradeClinical presentationTreatment
1Asymptomatic to local pain at bite site
Normal vital signs
Wound care, cold packs
Oral analgesia (i.e., nonsteroidal anti-inflammatory drugs, acetaminophen)
Tetanus vaccine prophylaxis (if indicated)
2Muscular pain in affected extremity extending proximally
Diaphoresis of bite site or affected extremity
Normal vital signs
Above treatments plus:
 Oral analgesia with or without parenteral opioids
 Oral or parenteral benzodiazepines for muscle spasm
 Antiemetic therapy
3Generalized muscular pain in back, abdomen, and chest
Diaphoresis remote from envenomation site
Nausea or vomiting, headache
Abnormal vital signs (hypertension, tachycardia)
Above treatments plus:
 Hospital admission
 Parenteral opioids
 Parenteral benzodiazepines
 Consider antivenom (if available) if refractory to supportive measures (especially in children and older people with comorbidities)

RECLUSE SPIDERS

Recluse spiders are found predominantly in the Southwest and southern Midwest and are best known for the brown recluse (Loxosceles reclusa), which are small (2 cm or less) brown spiders with a dark violin-shaped pattern on the anterior thorax (Figure 2). Many harmless spiders look similar and are frequently misidentified as brown recluses. Recluses are most commonly encountered indoors in dark, quiet areas such as furniture, clothes, and bedsheets. Recluse spiders are not aggressive by nature and only bite when antagonized, usually on the trunk if the patient inadvertently rolls onto or traps the spider in clothes. Bites typically resolve in one week or less, but an estimated 10% become necrotic after 24 to 48 hours, which may require weeks to months to heal and can result in scarring. Brown recluse bites are distinguished from other spider bites by the presence of a single, flat lesion without associated swelling.10

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